The present invention is directed to tangential fan cutoff designs that reduce blade passing, frequency tonal sound levels.
In a typical arrangement having a tangential fan wheel, a scroll housing and a cutoff, the cutoff gap between the cutoff and the fan wheel is a critical dimension relative to the fan's airflow performance capability. Smaller fan cutoff gaps yield higher airflow, while larger fan cutoff gaps yield lower airflow. However, for traditional cutoff designs such as the design shown in FIG. 1, optimum airflow performance arrangements having a smaller cutoff gap also result in a significant and objectionable blade tone. The acoustic strength of the blade tone is a function of the blade spacing, the cutoff gap size, the scroll shape, and the cutoff design. The blade tone can be reduced by increasing the cutoff gap spacing, but at the cost of reduced airflow performance. An optimum spacing of the fan cutoff gap is shown by the formula G=KD where G is the cutoff gap size, D is the fan wheel diameter, and K ranges between 0.038 and 0.055.
Another method of reducing the blade tone is to increase the number of fan blades. However, this increases fan cost and reduces airflow performance due to the increased number of blades blocking the fan flow passage.